I'm assuming this is a jack with the charger in the middle and two batteries on each side. Be sure that all four batteries have enough water to cover the cells before trying to charge. (if you're using flooded cell lead acid batteries)

The onboard charger is a completely separate unit from the pallet jack wiring. If your charger is bad, the batteries might be too low to run the jack. Measure the battery voltage at the connector both when trying to run and when trying to charge. It should charge at at least 25.5 volts. When trying to run it shouldn't drop below about 20 volts.